Practical Developments Using Today's Fractional Synthesizers

Integrated fractional PLL solutions have been available since the late 1990s. Over the past decade the in-band phase noise associated with the fractional processes has been greatly reduced. So have the troublesome fractional spurs. The base noise floor in dBc/Hz for some state of the art, low noise fractional PLLs approaches that of the lowest integer devices. Table 1 shows data for the recently released ADF4150 and the LMX2487E, which was recently reevaluated and found to have a lower noise floor then what’s on the datasheet. The table doesn’t take into account PLL 1/f noise. 1/f noise is a phenomena that increases by 3 dB/octave at some break point close to the carrier frequency, i.e. 1 to 10 kHz. For low noise, narrow span applications this may be a concern.